The production of fluorinated organic compounds, such as HFCs and HCFCs, is well known in the art. Among the more widely-used fluorination methods is liquid-phase, catalytic fluorination which is of particular interest herein. In this type of fluorination, a chlorinated organic precursor is reacted with anhydrous hydrogen fluoride (HF) in a reactor in the presence of a catalyst and under conditions sufficient to form a fluorinated organic compound. The fluorinated product tends to be more volatile then the chlorinated precursor, and is separated from the reaction mixture by distillation, along with the principal byproduct, hydrogen chloride (HCl).
Although widely used, conventional liquid-phase fluorination suffers from several shortcomings, one of the more significant being the compromise between high reaction rates and extensive corrosion to the reactor vessel. More specifically, it is common for a high concentration of highly-corrosive catalyst to be used as a reaction solvent. With such a high concentration, specific reaction conditions must be maintained to avoid formation of unwanted byproducts and extensive corrosion of the reactor vessel, even those comprising corrosion-resistant alloys such as Inconel and Hastalloy. These specific reaction conditions, however, tend to be outside the range generally preferred for optimum reaction rates. Additionally, the concentration of HF must be minimized because the corrosivity of the reaction mixture increases dramatically with higher HF concentrations. Unfortunately, however, lower concentrations of HF also result in lower reaction rates and production suffers.
Aside from its corrosive effects, maintaining such a high concentration of catalyst also tends to be expensive, thus favoring the use of chlorinated catalysts, such as antimony pentachloride, which are less expensive, but also less effective than their fluorinated counterparts. Chlorinated catalysts also tend to undergo deactivation to a less reactive state under commonly-used reaction conditions. For example, at temperatures of about 90.degree. to about 110.degree. C., antimony (+V) halides will deactivate into a (+III) halide. The deactivation results in improper fluorination and a general reduction in product selectivity. To counteract the catalyst's deactivation, an oxidizing agent, such as chlorine, is added to regenerate the catalyst. The addition of chlorine, however, tends to contribute to corrosion. Additionally, excess chlorine in the reaction mixture promotes side reactions which produce by-products. Therefore, the addition of chlorine increases reactor corrosion and generates unwanted by-products.
Reactor corrosion can be reduced by using the fluorinated product as the reaction solvent. However, given the relatively-high volatilities of the fluorinated products, such as, HFC-41, 23, 32, and 143a, extremely high reaction pressures result under normal operating conditions. Such high pressures necessitate the use of high pressure-rated reactors (for example, rated for pressures greater than 500 psig) which can be prohibitively expensive.
Therefore, a need exists for a fluorination process that is less corrosive, generates fewer by-products and avoids the need for oxidizing agents and high reaction pressures. The present invention fulfils this need among others.